Method of producing a zirconium alloy semi-finished product for the production of elongated product and use thereof

ABSTRACT

A large ingot is produced by casting the zirconium alloy, then the ingot is forged in two stages to obtain the semi-finished product wherein the first stage of forging the ingot is performed at a temperature at which the zirconium alloy is in a state comprising crystalline α and β phases.

FIELD OF THE INVENTION

The present invention concerns a method of producing a zirconium alloysemi-finished product intended for the production of an elongatedproduct used for the manufacture of fuel assembly elements.

BACKGROUND INFORMATION

Fuel assemblies in nuclear reactors cooled by light water, for examplepressurized water reactors (PWR) and boiling water reactors (BWR), orfuel assemblies of CANDU reactors, contain elements comprising azirconium alloy with the property of low neutron absorption in the heartof the nuclear reactor.

In the case of assemblies for PWR-type nuclear reactors, the jackettubes for the fuel rods and the plates used for production of the spacergrids for the fuel assembly can be made of zirconium alloy, inparticular zirconium alloy containing tin, iron, chromium and whereapplicable nickel, such as alloys Zircaloy 2 or Zircaloy 4. The sameapplies to the plugs which close the jacket rods at both ends.

Other alloys, such as the alloy known under the commercial name M5,essentially comprising zirconium and niobium are also used for theproduction of fuel assembly elements in the form of flat or elongated,solid or tubular products.

In general, the zirconium alloys used for the production of fuelassembly elements comprise at least 97% zirconium by weight, theremainder of the composition which represents at most 3% by weight, withthe exception of impurities due to the production of the alloy, cancomprise various elements and in particular iron, tin or niobium.

Zirconium alloys meeting these conditions in relation to theircomposition, depending on the temperature and the heat treatment towhich they are subjected, can take one or the other of the twoallotropic forms of zirconium i.e. the alpha phase, which is the phaseof zirconium stable at low temperature with a compact hexagonalstructure, or the beta phase, which is the phase stable at hightemperature with a cubic structure. In certain temperature ranges or atthe end of certain treatments, zirconium alloys such as the technicalalloys used for the production of fuel assembly rods defined above canhave a mixed alpha+beta structure.

Tubular products of zirconium alloy are generally produced by extrusionof a rod which is itself obtained from an ingot by forming and whereapplicable machining operations.

Solid elongated products (bars) are generally produced by hot rollingthen cold hammering of the semi-finished products obtained from theingot.

Normally a large ingot is cast with a diameter for example between 400and 700 mm, and generally between 600 and 660 mm. The ingot thenundergoes the forging operations in a temperature range in which it canbe in the α, β or α+β phase (EP-0.085.552 and U.S. Pat. No. 5,674,330).The ingot is β-phase forged at a temperature between 1000° C. and 1100°C., generally around 1050° C. in the case of Zircaloy 4, to obtain anintermediate product such as a bar or a product of square or octagonalsection, of which the diameter of the transverse section (or thediameter of the circle circumscribing the transverse section) is between250 mm and 400 mm. For example, in the case of an octagonal section,this can have a diagonal with length of the order of 350 mm whichcorresponds to the diameter of the circle circumscribed.

The intermediate product is then α-phase forged at a temperature between700° C. and 800° C., for example typically at 750° C., until a bar isobtained with a diameter of 100 mm to 250 mm (and typically a diameterof 205 mm).

Then either the bar resulting from the previous forging phase or a blockcomprising a part of a cut bar, or a rod produced from a block drilledin its axial direction, is hardened from the β phase (typically from atemperature between 1000° C. and 1150° C.).

Finally, to obtain a tubular product a rod is extruded which can eitherbe the hardened rod obtained in the preceding phase or a rod machinedfrom a hardened bar obtained during the preceding phase of theproduction process.

To obtain an elongated solid product, hot rolling is performed on thehardened bar.

In all cases before the extrusion operation creating the final tubularproduct or the hot rolling operation creating a small diameter bar, asemi-finished product is produced in the form of a bar, a block or a rodby a production process comprising a first stage of β-phase forging ofthe starting ingot and a second stage of α-phase forging of theintermediate product obtained at the end of the first β-phase forgingstage.

The known transformation process which has just been described comprisesa first β-phase forging stage at a high temperature between 1000° C. and1100° C. After this first forging stage the intermediate productobtained is cooled at least to the temperature for α-phase forging andgenerally to ambient temperature because the second α-phase forgingstage is not performed immediately after the first β-phase forgingstage.

The very high temperature forging of the ingot is a costly and delicateprocess.

Also during heating of the ingot to bring it to a temperature of 1000°C. to 1100° C. before the first forging stage, the intermediate ingotcan absorb hydrogen from contact with humid air or water, the hydrogenfixing in the material in the form of hydrides.

In general the presence of hydrides in the material in the form ofcoarse precipitates is harmful to the cold formability and corrosionresistance of the products.

SUMMARY

The objective of the invention is to propose a production process for azirconium alloy semi-finished product containing by weight at least 97%zirconium and intended for the production of at least one elongatedproduct, in which a large ingot is produced by casting the zirconiumalloy, then a semi-finished product intended to be formed to obtain theelongated product is produced by two-stage forging of the large ingot,where this method simplifies and reduces the cost of production of theelongated product, and limits the presence of hydrides to low levels inthe semi-finished product and hence in the elongated end product.

To this end the first stage of forging the large ingot is performed at atemperature at which the zirconium alloy is in a state comprisingcrystalline α and β phases of the zirconium alloy.

According to particular features:

-   -   at the temperature of the first forging stage, the ingot        comprises a volume proportion of zirconium alloy in the a phase        between 10% and 90%, the rest of the zirconium alloy of the        ingot being in the β phase,    -   the first forging stage is performed at a temperature between        850° C. and 950° C.;    -   the first forging stage is performed at a temperature around        900° C.;    -   the first forging stage is performed at a temperature between        600° C. and 950° C.;    -   the second forging stage is performed at a temperature at which        the zirconium alloy of an intermediate product obtained from the        first stage of forging the ingot is in the a phase;    -   the second forging stage is performed at a temperature at which        the zirconium alloy of an intermediate product obtained on        completion of the first stage of forging the ingot is in a state        comprising crystalline α and β phases of the zirconium alloy;        and    -   the zirconium alloy comprises at least 3% by weight in total of        additional elements comprising at least one of the elements tin,        iron, chromium, nickel, oxygen, niobium, vanadium and silicon,        the remainder of the alloy being constituted by zirconium with        the exception of the inevitable impurities.

The invention also relates to:

-   -   the use of the method for production of a semi-finished product        such as a bar or rod intended for production of a tubular        product for manufacture of a fuel assembly element such as a        jacket tube or guide tube of a fuel assembly for a water-cooled        nuclear reactor or a fuel assembly element for a CANDU reactor;    -   or the use of the method for production of a bar intended for        production of a small diameter plug bar for the manufacture of        plugs for closing the ends of the jacket tubes of the fuel        assembly rods for the nuclear reactor.

In order to understand the invention, a production method will bedescribed of a semi-finished product intended for production of tubularproducts according to the invention, by comparison with the methodaccording to the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing in a simplified manner the various stages ofthe method of production of the semi-finished product.

DETAILED DESCRIPTION

FIG. 1 shows a cast ingot 1 which can be a large ingot, the diameter ofwhich can be between 400 mm and 700 mm and the length between 2 m and 3m, which is obtained by casting a zirconium alloy used for theproduction of tubular products for the manufacture of fuel assemblyelements.

The zirconium alloy can be for example a Zircaloy 2 alloy comprising inweight from 1.2% to 1.7% tin, 0.07% to 0.20% iron, 0.05% to 0.15%chromium, 0.03% to 0.08% nickel, at most 120 ppm silicon and 150 ppmcarbon, the remainder of the alloy being constituted by zirconium withthe exception of the usual impurities.

The alloy for production of the elongated product can be also a Zircaloy4 comprising in weight 1.2% to 1.7% tin, 0.18% to 0.24% iron, 0.07% to0.13% chromium, at most 150 ppm carbon, the remainder of the alloy beingconstituted by zirconium and impurities.

The zirconium alloy used to make the elongated product can also be analloy of the M5 type comprising essentially zirconium and niobium.

According to the invention the ingot is brought to a temperature atwhich the zirconium alloy is in the α+β phase, to perform the firststage of forging the ingot in the α+β phase.

The temperature for the α+β phase forging (first stage of the method) isselected so that the volume proportion of the α phase in the ingot alloyis between 10% and 90%, the remainder of the alloy being in the β phase.

Generally the first forging stage is performed at a temperature between850° C. and 950° C. and for example typically at 900° C. in the case ofZircaloy 4. At this temperature the zirconium alloys such as Zircaloyare in the α+β phase. In the case of zirconium-niobium alloys such asalloy M5, the α+β phase extends over a temperature range substantiallylarger than in the case of the Zircaloy type alloys, this range beingfrom 600° C. to 950° C.

The ingot is forged as in the case of the method according toconventional methods, where the forging was performed at hightemperature (for example 1050° C.) until a bar is obtained or a productof square or octagonal section inscribed in a circle of diameter 250 mmto 400 mm, typically in a circle of diameter of 350 mm.

Substitution of α+β phase forging for β-phase forging at a highertemperature gives an intermediate product with characteristics similarto those of the usual intermediate product obtained by the first β-phaseforging stage.

The lowering of the forging temperature, for example to 150° C., isreflected in substantial savings in implementation of the productionprocess.

Also this forging can be performed using conventional tooling takinginto account a slight adaptation of the forging process.

In the case of zirconium-niobium alloys such as M5, it is possible toperform the forging at a temperature substantially lower than 900° C.,the α+β phase of the alloy extending from the temperature of 600° C. to950° C.

In a first variant of the invention, the second stage of the forgingprocess to obtain the semi-finished product from an intermediate productcan be performed in the same manner as in the known process inconventional methods, i.e. performing a second forging in the a phase ata temperature between 700° C. and 800° C. to give a bar with a diameterbetween 100 mm and 250 mm.

In a second variant of the process it is possible to perform the secondforging stage, to obtain a semi-finished product in the form of a bar,at the same temperature as the first forging stage i.e. on the productin the α+β phase.

The diagram shows schematically the forging installation allowingperformance of the first forging stage 2 on the ingot 1 at a temperatureat which the ingot 1 is in the α+β phase. After the first forging stage2, an intermediate product 3′ is obtained comprising a bar or a productof square or octagonal section which is subjected to a second forgingstage 4 to obtain the semi-finished product 3 in the form of a rod orbar, from which the elongated end product can be obtained by extrusionor hot rolling.

The tooling used in the first α+β phase forging stage 2 and in thesecond forging stage 4 can be conventional tools used as part of aprocess of the conventional methods in which the first stage 2 isperformed on the ingot 1 in the β phase and the second stage 4 on theintermediate product 3′ in the a phase.

In the case of the invention, the second forging stage 2 can beperformed at the same temperature as the first forging stage 2, theintermediate product 3′ being in the α+β phase.

The second stage 4 can also be performed in the a phase as in the caseof a process of conventional methods.

The intermediate product 3′ obtained from the first forging stage in theα+β phase can be subjected to a cooling stage of any type.

The intermediate product 3′ can be immediately brought to thetemperature of the second forging stage i.e. a temperature at which theproduct is in the a phase or in the α+β phase.

In the case where the two forging stages are performed in the α+β phase,the product temperature can be maintained between the two forgingstages.

Forging the ingot 1 in two stages allows production of a bar or rod witha diameter between 100 and 250 mm that constitutes the semi-finishedproduct which is then subjected to the operation of extrusion or hotrolling to obtain a tubular part or a small diameter bar that can beused for production of elements for fuel assemblies for nuclearreactors.

By performing analyses on the semi-finished product 3 or on theelongated products obtained from the semi-finished product, it can beobserved that the quantity of hydrides contained in the alloy obtainedby the process according to the invention is substantially smaller thanthe quantity of hydrides contained in a product according toconventional methods.

Also the semi-finished product or elongated end products obtained fromthis semi-finished product have mechanical and structuralcharacteristics substantially similar to those of products obtained by aprocess according to conventional methods.

In particular the corrosion resistance and formability of the tubularproducts produced from the semi-finished product according to theinvention are substantially superior to those of the product obtained bythe process according to the conventional methods.

One of the advantages of the method according to the invention is tosimplify the process of production of the semi-finished product bylimiting the forging temperature during the first forging stage andwhere applicable omitting any cooling after the first forging stage.This reduces the cost and duration of the implementation of the method.

In the case of the Zircaloy 2 and Zircaloy 4 alloys or any otherzirconium alloy containing tin, the transition to the α+β phase of thealloy, to perform the first stage and where applicable the second stageof the method according to the invention, can lead to the formation oftin segregations. However, these segregations can be suppressed bysubsequent processing within the context of production of the tubularend product from the semi-finished product. The same applies to theelements oxygen and nitrogen.

In the case where the process of the invention is applied to niobiumalloys as indicated above, the transition between the α and α+β phasesbeing close to 600° C., the forging temperature in the α+β phase can besubstantially lower than 900° C. taking into account however themalleability properties of the alloy at the forging temperature.

Application of the process according to the invention to zirconiumalloys other than Zircaloy or to niobium alloys can be considered. Thesealloys generally contain at most 3% in weight of additive elementscomprising at least one of the additive elements tin, iron, chromium,nickel, oxygen, niobium, vanadium and silicon, the remainder of thealloy being constituted by zirconium and the inevitable impurities.

The invention applies in particular to the production of tubularproducts of zirconium alloy for the manufacture of fuel assemblyelements such as jacket tubes containing fuel pellets or guide tubes forfuel assemblies.

The invention also applies to the production of plug bars for themanufacture of plugs closing the ends of the jacket tubes of fuelassembly rods.

To obtain the end products from the semi-finished product, it may benecessary to perform operations after the extrusion or hot rolling ofthe semi-finished product, such as pilger rolling, where heat treatmentscan be also performed between the forming operations.

The invention is not limited strictly to the embodiments described.

The temperature for forging in the α+β phase depends on the compositionof the zirconium alloy. The forming operations can be performed usingthe normal air augmented for forming in the α phase or β phase of theprocess of the conventional methods or other arrangements and methodsadapted to α+β phase forging in one or two stages to obtain thesemi-finished product.

The invention applies generally to any technical zirconium alloy productdefined by the composition limits given above.

1-10. (canceled)
 11. A method for producing a zirconium alloysemi-finished product containing by weight at least 97% zirconium,intended for the production of at least one elongated product,comprising: casting the zirconium alloy to produce an ingot with adiameter between 400 mm and 700 mm and a length between 2 m and 3 m; andtwo-stage forging the ingot to produce the semi-finished productintended to be formed to obtain the elongated product, wherein a firstforging stage of the ingot is performed at a temperature at which thezirconium alloy is in a state comprising the crystalline α and β phasesof the zirconium alloy.
 12. The method according to claim 11, wherein atthe temperature of the first forging stage, the ingot contains a volumeproportion of zirconium alloy in the a phase between 10% and 90%, aremainder of the zirconium alloy of the ingot being in the β phase. 13.The method according to claim 11, wherein the first forging stage isperformed at a temperature between 850° C. and 950° C.
 14. The methodaccording to claim 13, wherein the first forging stage is performed at atemperature of approximately 900° C.
 15. The method according to claim11, wherein the first forging stage is performed at a temperaturebetween 600° C. and 950° C.
 16. The method according to claim 11,further comprising: performing a second forging stage at a temperatureat which the zirconium alloy of an intermediate product obtained by thefirst forging stage of the ingot is in the a phase.
 17. The method asclaimed in claim 11, wherein a second forging stage is performed at atemperature at which the zirconium alloy of an intermediate productobtained at an end of the first forging stage of the ingot is in a statecomprising crystalline α and β phases of the zirconium alloy.
 18. Themethod according to claim 11, wherein the zirconium alloy contains atleast 3% by weight in total of additive elements comprising at least oneof tin, iron, chromium, nickel, oxygen, niobium, vanadium and silicon, aremainder of the alloy being constituted by zirconium with an exceptionof the inevitable impurities.
 19. The method according to claim 11further comprising: producing a semi-finished product intended forproduction of a tubular product for manufacture of a fuel assemblyelement for one of a fuel assembly for a water-cooled nuclear reactorand a fuel assembly element for a CANDU reactor.
 20. The methodaccording to claim 11 further comprising: producing a bar intended forproduction of a small diameter plug bar for manufacture of plugs closingends of jacket tubes of fuel assembly rods for nuclear reactors.